The present invention relates to a fixing apparatus which controls power to be supplied to a heater serving as a heat source for fixing used in a copying machine or a printer, and to an electrophotographic apparatus having therein the fixing apparatus mentioned above. It further relates, in particular, to a fixing apparatus and an electrophotographic apparatus which give consideration to flicker caused by a voltage fall that is caused in the surrounding area by an electric current running through a heater.
In general, in an image forming apparatus of an electrophotographic type, image information (original image) of an original is converted to electrical signals (image signals) corresponding to density of the image information, and based on the image signals, an electrostatic latent image is formed by a laser beam or the like on a photoreceptor drum. This electrostatic latent image is developed through development to a toner image which is transferred onto a recording sheet. Then, the toner image on the recording sheet is heated by a heater in a fixing apparatus to be fused and fixed.
As a heater (fixing heater) of the fixing apparatus mentioned above, a heater represented by a halogen lamp or the like is used as a heat source, and such fixing heater is housed in a heat roller.
As a heater of this kind, 100-1000 w fixing heaters are used in a small-sized image forming apparatus, and a fixing heater having a greater wattage is used for those wherein images are formed at high speed.
Incidentally, with regard to the number of heaters, there are various cases including one, two or three heaters.
A fixing heater is controlled so that ON/OFF of power supply to the fixing heater can be controlled in accordance with heater ON signals generated based on results of detection of a temperature sensor arranged in the vicinity of a heat roller and thereby the constant fixing temperature may be maintained.
In the fixing apparatus of this kind, a large rush current flows momentarily at the moment when power supply to a fixing heater changes from OFF to ON.
The foregoing will be explained as follows, referring to FIG. 18. FIG. 18(a) shows a voltage waveform of commercial power supply (A.C. 100V) .
At a certain timing, a heater ON signal is changed to the state of ON (FIG. 18(b)), and A.C. 100V is supplied to a halogen heater from commercial power supply.
A resistance value of the halogen heater to which no electric current has been supplied up to that moment is extremely low, and its value is about one tenth of the resistance value in the state of red heat, generally.
Accordingly, rush current I' flows in the halogen heater because the electric current starts flowing for a low resistance value simultaneously with power supply to the halogen heater (FIG. 18(c)). Then, as the resistance value rises to the regular value, the heater current falls to I to be converged. Assuming, for example, that the resistance value of the halogen heater being in the state of OFF is one tenth of that in the state of red heat, if heater ON signals are changed to the state of ON when voltage of commercial power supply is high, an electric current which is about ten times greater (I'=10I) is supposed to flow.
When such rush current I' is generated, a voltage fall (V1) is caused by electric resistance (impedance) in a receptacle of a commercial power supply that supplies power to an image forming apparatus or in the surrounding thereof or in interior wiring. After that, when the heater current is converged to I, voltage of power supply is slightly restored.
The foregoing will be explained as follows, referring to FIG. 19 which shows a waveform of a peak value of voltage. In this case, the heater is turned ON at the time t1 and rush current is generated, resulting in an outbreak of momentary large voltage fall. After that, the voltage fall is converged to a small value (constant value). Then, the heater is turned OFF at the time of t2, and voltage is returned to its original level.
Since the voltage fall caused by the aforementioned rush current is momentarily great in scale, in particular, it sometimes has an influence even on surrounding equipment and lighting apparatus. For example, when voltage supplied to lighting apparatus is lowered, there sometimes occurs a phenomenon called flicker which means that illuminance is momentarily lowered.
In the case of the foregoing, the heater ON signal was turned ON under the state of high A.C. voltage, and this is why a large rush current flowed. It can be considered therefore that a zero-cross circuit is provided, and that the heater ON signal is turned ON at the timing when power supply voltage is 0V. Owing to this arrangement, it is possible to control a value of rush current to be small because a resistance of the heater rises to a certain extent before the voltage of the heater reaches its peak value.
When such zero-cross control is conducted, the heater ON signal is turned ON at the timing in FIG. 18(d), and electric current flows as shown in FIG. 18(e). In this case, since the heater ON signal is turned ON at the timing when a peak value of power supply is 0V, the rush current at that timing of ON is smaller than that described before.
Namely, when experiments were made under a certain condition, rush current I' was about 5I which was a half in terms of value of the rush current described above.
However, rush current that is greater than a regular current still flows, and flicker caused by voltage fall still occurs.
It is therefore considered, for preventing rush current, that two heaters are used to be turned on one by one on a stepwise basis.
However, the method mentioned above requires two systems of control circuits for controlling the two heaters, and a diameter of a fixing roller needs to be sufficient for housing therein two heaters. Therefore, the production cost is increased, which is a problem. In addition, this method can not be applied to a fixing roller designed originally to house one heater.
It is also considered that a resistor or a thermistor is provided in series with a heater, and for a certain period from ON, the heater and the resistor or the thermistor are connected in series to be energized, and then the resistor or the thermistor is cut off and the heater is energized. However, this method has many problems including a problem of heat generated from the heater or other sources, a problem of loss (efficiency drop) caused by the resistor or other sources, and a problem of reliability of circuits for executing connection/cutting off.
For preventing rush current as mentioned above, therefore, there sometimes is used a circuit called a soft starter circuit employing bi-directional and 3-terminal thyristor and conducting continuity angle control.
FIGS. 20(a) and 20(b) represent time charts showing waveforms in a soft starter circuit of this kind, wherein FIG. 20(a) shows waveforms of power supply voltage and FIG. 20(b) shows waveforms of a current controlled in terms of continuity angle. Incidentally, indications in this case are based on an assumption that there is no phase difference between voltage and current.
In FIG. 20(b), a solid line represents a period of time when the bi-directional and 3-terminal thyristor is actually made to be in the state of continuity. Namely, occurrence of rush current is inhibited when the continuity angle (period of continuity in a half cycle) is increased gradually.
In the case of this soft starter circuit, the rise at the moment of continuity in each cycle shows a sharp waveform as shown in FIG. 20(b). Therefore, noise is radiated over a wide frequency range, jamming TV or radio reception. Accordingly, to comply with regulations of noise stipulated as a terminal noise standard, it is necessary to provide a noise filter on a power supply line, which causes a problem of cost increase.
As explained above, occurrence of flicker caused by rush current and a problem of cost for preventing noise are on the relation of a trade-off, and realization of an inexpensive apparatus generating no flicker has been desired.
The present invention has been attained in consideration of the aforementioned problems, and its first object is to realize a heater control device capable of restraining an influence of flicker caused by voltage fall of power supply that results from rush current.
Further, FIG. 21 is a block diagram showing the constitution of power supply lines located in the vicinity of a fixing apparatus in the case of conducting continuity angle control by a thyristor. In FIG. 21, power is supplied from commercial power supply through power plug 11, and noise coming from fixing apparatus 40 is prevented from leaking in commercial power supply by noise filter 20. In this case, the noise filter 20 is composed of common choke 21, X capacitor 22 and Y capacitors 23 and 24. Incidentally, D.C. power supply 30 is a power supply that supplies prescribed D.C. voltage to each section in an apparatus (unillustrated process means such as a charging means, a developing means and a transfer section).
For example, in the constitution shown in FIG. 21, electric current of about 8-10 A flows to fixing apparatus 40 which includes heater 41, and that of about 1-2 A flows to D.C. power supply 30. Therefore, noise filter 20 is also constituted to be a large filter that withstands large current of about 12 A. In particular, a common choke is unavoidably required to be large in size, which leads to a large-sized apparatus.
In particular, when odd-number-order harmonic current generated in D.C. power supply 30 and that generated from fixing apparatus 40 are superposed, it sometimes happens that a level of noise generated from an entire apparatus exceeds a range of the noise standard even when each of the aforesaid D.C. power supply and fixing apparatus.
Here, the odd-number-order harmonic current generated in D.C. power supply 30 will be explained briefly. The D.C. power supply 30 is of a circuit structure shown in FIG. 22, for example, wherein an electric current rectified by diode bridge 31 charges electrolytic capacitor 32, then switching is made by SW element 33, and necessary voltage is outputted from SW transformer 34.
In this case, current i from a commercial power supply flows in the state of pulses only when voltage V nears V0, because the electrolytic capacitor 32 is charged almost to peak voltage V0. Since this current waveform (see FIG. 23) is symmetrical in terms of positive and negative sides, it is understood that Fourier spectrum takes only odd-number-order harmonic.
For the reason mentioned above, it is unavoidable that a common choke be made large, which is a problem.
The present invention has been achieved in consideration of the foregoing, and its second object is to realize a fixing apparatus or an electrophotographic apparatus which employs a halogen lamp heater as a heat source and which is capable of overcoming the problems of flicker and noise by a simple circuit arrangement without controlling a continuity angle.